ci: added treefmt workflow to gha and removed from buildbot

[c4patino]

Seemed like low hanging fruit in #3561

• Migrates the treefmt foramtting check from buildbot to a dedicated Github Actions workflow.
• Removes the buildbot treefmt check from flake/dev/fmt.nix to avoid duplicate runs with the new formatter.

[MattSturgeon]

Thanks for working on this!

[c4patino]

Hi @MattSturgeon is there anything else that you need on this?

[MattSturgeon]

This is complicated a little by merge queues and "required status checks", however I'm fine merging this initially without adding it as a "required" check.

#3628 can then be responsible for implementing the required checks for PRs and merge queues.

A few minor things to address or discuss below, then I think we can get this merged.

Thanks again!

[MattSturgeon]

LGTM, thanks!